Update jekyll requirement from ~> 3.8.5 to >= 3.8.5, < 4.1.0

Updates the requirements on [jekyll](https://github.com/jekyll/jekyll) to permit the latest version.
- [Release notes](https://github.com/jekyll/jekyll/releases)
- [Changelog](https://github.com/jekyll/jekyll/blob/master/History.markdown)
- [Commits](https://github.com/jekyll/jekyll/compare/v3.8.5...v4.0.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

just-the-docs.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|bin|_layouts|_includes|lib|Rakefile|_sass|LICENSE|README)}i) }
   spec.executables   << 'just-the-docs'
 
-  spec.add_runtime_dependency "jekyll", "~> 3.8.5"
+  spec.add_runtime_dependency "jekyll", ">= 3.8.5", "< 4.1.0"
   spec.add_runtime_dependency "jekyll-seo-tag", "~> 2.0"
   spec.add_runtime_dependency "rake", "~> 12.3.1"